Flame spraying has become the preferred process in a number of applications, particularly in the field of high-performance coatings. In general, flame spray technology relies on the use of specially designed spray guns which project a stream of high-temperature particles onto the surface of a substrate. A number of materials have been successfully flame sprayed, including metals, oxides, and cermets, as well as some glasses and plastics. Depending upon the spray gun design, the feedstock can be supplied as a powder, wire, or rod or as combination of these forms. Where a powdered feedstock is used, the powder is typically held in a hopper and is fed to the spray gun by gravity feed or, more preferably, by a carrier gas.
In operation, the feedstock is metered into the spray gun where it is heated and accelerated to form a particle stream having sufficient thermal and kinetic energy to form a dense, adherent coating on a preselected substrate. Heat is produced by combustion of a fuel gas. In one type of spray gun, fuel and oxygen streams combine to form a flame front in coaxial relation with the incoming feedstock stream. As the solid feedstock enters the high-temperature combustion flame, it is heated (and atomized where a non-particulate feedstock is used) to a temperature within a preselected temperature range.
A number of factors determine feedstock temperature, including residence time of the particles in the flame and flame temperature. In most applications, the feedstock particles must be melted or at least softened by the flame. A uniform temperature profile is desired, since inclusions of unmelted particles in the resultant coating may seriously reduce durability of the coating.
In addition to supplying heat to the feedstock, the combustion gases provide the force necessary to accelerate the molten feedstock particles to the high-velocities which are required to form high-quality coatings. In a number of spray guns, combustion occurs within a combustion chamber in the gun body. As the high-temperature combustion gases expand, they are directed through a spray nozzle to form a stream of high-velocity gas which accelerates the molten particles to velocities which may exceed twenty-five hundred feet per second. If particle velocity is too low, inferior coatings may be produced due to low impact forces by the particles on the substrate. Unwanted volitilization of the feedstock may also occur due to excessive residence time of the particles in the high-temperature region of the gun.
Upon impact, the high-velocity softened or molten particles impinge on the substrate and flatten to form thin platelets of material that conform to the substrate surface. The individual "splats" adhere to one another and form a mechanical and, often, a metallurgical bond with the substrate material. The deposited material then solidifies rapidly, forming a dense adherent coating.
Therefore, two fundamental performance requirements of any flame spray gun are the ability to provide requisite but not excessive particle temperature and the capacity to produce a minimum particle velocity to form high-density coatings. In addition, however, a performance requirement which has often been overlooked in conventional flame spray gun designs is the need for durability and simplicity of the gun. Although a number of flame spray gun prototypes may be capable of producing high-quality coatings under laboratory test conditions, the overall design of these guns may be such that they require constant calibration and maintenance. Moreover, a design which works well as a prototype that is machined and assembled by a skilled craftsman and may be difficult to produce on a large-scale basis. In particular, tolerances which can be met in the assembly of a prototype may be too restrictive in a production environment, resulting in a product which does not reliably reproduce the coatings obtained in prototype testing.
The present invention addresses these concerns by providing a flame spray gun which is unique in design and which can be easily manufactured and maintained--without sacrificing particle temperature and velocity control necessary to produce high-quality coatings.
By way of a survey of conventional flame spray guns, numerous guns have been identified in the patent literature. That of U.S. Pat. No. 4,416,421 is described as an "Ultra High-Velocity Flame Spray Apparatus" and includes a combustion chamber in which oxygen and a fuel gas are premixed and combusted to form combustion gases. The combustion gases are discharged through a nozzle. A feedstock is introduced at or upstream of the throat of an extended length nozzle. It is stated that the length of the nozzle bore should be substantially greater than the minimum diameter of the bore and that the pressure within the combustion chamber should be maintained at 75 PSIG or greater.
In U.S. Pat. No. 4,836,477, a relatively small diameter oxidizer inlet bore leads to a diverging conical diffuser which in turn leads to and merges with a larger diameter bore section. The large diameter bore defines a flame spray duct. It is claimed that the design allows all flow trajectories to pass in parallel through the duct bore, eliminating flow recirculation and particle recirculation. In U.S. Pat. No. 1,930,373, a spray gun is described in which concentric annuli are used to form a concial flame within a truncated gun nozzle. A feedstock is introduced into the flame axially through a central bore in the gun body.
In U.S. Pat. No. 2,125,764, a spray apparatus is disclosed in which jets of compressed air flow in the form of a ring around a flame and serve to propel powder. Powder is carried in a central passage through the body of the apparatus. U.S. Pat. No. 2,804,337 discloses a spray apparatus having a portion which includes a plurality of gas outlets surrounding a central bore through which particles are conveyed. In U.S. Pat. No. 4,302,483,a spray apparatus is disclosed in which gases are conveyed in concentric annuli around an axial feedstock bore.
In U.S. Pat No. 4,562,961, a gunitting nozzle end piece is disclosed which includes a plurality of individual tubes disposed in ring-like fashion around an internal ring comprised of a second set of tubes. A refractory powder is carried through at least one set of the plurality of tubes. U.S. Pat. No. 4,634,611 discloses a flame spray apparatus wherein an oxygen and fuel gas mixture is ignited in a straight bore combustion throat into which a feedstock is added through an axial bore.